1. Field of the Invention
This invention relates to a process of chromatographic separation and more particularly to a method of controlling chromatographic separation of a fluid mixture comprising two or more components into two or more fractions enriched in the respective components.
2. Description of Related Art
Chromatographic separation by use of a solid adsorbent is extensively conducted in industries. Various techniques of chromatographic separation have been proposed for separating a mixture of two or more components into fractions enriched in each component.
Among chromatographic separation systems a simulated moving-bed system is widely used in industries for its excellent separation performance and high productivity. In this system a feedstock fluid or a desorbent is supplied to a packed bed at a prescribed constant flow rate, and the fluid flows through the packed bed also at a prescribed constant flow rate (hereinafter referred to as a constant flow rate control system) The simulated moving-bed system, however, requires a complicated apparatus and high skill of control on the supply of a feedstock fluid or a desorbent to the packed bed and on the movement of the fluid circulating through the packed bed.
Chromatographic separation processes which can achieve satisfactory separation results with simpler apparatus have been proposed as disclosed in JP-A-63-158105 (corresponding to U.S. Pat. No. 4,970,002 and Canadian Patent No. 1305434) and JP-A-2-49159. The process disclosed in JP-A-63-158105, for example, comprises repetition of cycles each including at least three steps; a step of supplying a feedstock fluid, a step of supplying a desorbent fluid, and a step of circulating the fluid through the packed bed.
In the simulated moving-bed system, the concentration distribution curves made in the packed bed macroscopically have almost the same form and circulatively move through the packed bed with time while keeping the form. Accordingly, the pressure required for moving the fluid through the packed bed, namely, the pressure drop (pressure loss) produced when the liquid moves through the packed bed may be seen substantially equal in any time zone. In this situation, the above-described constant flow rate control system is an effective system for controlling the apparatus with good reproducibility to obtain desired separation performance.
According to the method of JP-A-63-158105, the flow rate of the feedstock liquid and the desorbent fluid while being supplied and the flow rate of the liquid moving through the packed bed are controlled at a prescribed rate, and switches among the steps are made for a certain lapse of time. In this case, however, the concentrations of the individual components in the packed bed and the concentration distributions formed in the bed gradually vary with time in every step. More specifically, in the step wherein the feedstock fluid is supplied while a fraction enriched in a certain component is withdrawn, the concentration of the components present in the packed bed gradually increases from start to finish of the supply. In the step wherein the desorbent is supplied while a fraction enriched in another certain component is withdrawn, the concentration of the component present in the packed bed gradually decreases from start to finish of the supply. Even in the step of circulating the fluid in the packed bed without supplying or withdrawing any liquid thereby to allow an adsorption zone containing a plurality of components to move to the upstream end of the packed bed, the concentration distribution in the packed bed gradually changes from start to finish of the step.
A mixture of saccharides (i.e., a carbohydrate solution including various kinds of sugars and/or sugar alcohols) is one of the most common feedstock fluid to be treated by chromatographic separation. In the present invention, the mixture of saccharides means a mixture solution of at least two selected from the group consisting of sugars and sugar alcohols. A mixture of saccharides greatly varies its viscosity depending on the concentration, and a high concentration mixture has a high viscosity. In treating such a fluid as greatly varies its viscosity according to the concentration, such as a saccharides mixture, variations in concentrations or concentration distributions of the components present in the packed bed necessitate variations of the pressure for moving the fluid through the bed. In other words, the pressure drop generated in moving the fluid through the packed bed gradually changes.
The changes in pressure drop are analyzed as follows. In the supply step in which a feedstock fluid containing a plurality of components is fed, a fluid having a lower concentration than the feedstock fluid and enriched in a certain component is withdrawn. Accordingly, the average concentration of the components in the bed gradually increases during this step. In the step of supplying the desorbent and withdrawing another fraction enriched in another component, the fluid withdrawn is obviously higher in concentration than the desorbent. This means that the average concentration of the components present in the packed bed is gradually decreasing in this desorption step.
Hence, the pressure necessary to allow the fluid to move through the packed bed generally reaches the maximum at about the end of the step of feeding the feedstock fluid and the minimum at about the end of the step of feeding the desorbent fluid.
In the chromatographic process described in JP-A-63-158105, a pump is used for supplying the feedstock liquid and the desorbent fluid and for circulatively moving the fluid through the packed bed. The operation is controlled by adjusting the flow rate of the pump so as to maintain the flow rate constant. The pump should be designed to have a discharged head to give at least the maximum pressure around the end of the feedstock supplying step. The discharged head required in the step of feeding the desorbent, on the other hand, does not need to be so high as in the end of the feedstock supplying step. While the desorbent is supplied, therefore, excess pressure is consumed by a control valve, etc. to keep the prescribed flow rate.
The column (container) containing the packed bed is also designed to have resistance to the maximum pressure imposed at about the end of the feedstock supplying step, but the same pressure resistance is not needed in the desorbent supplying step.
In this way, since the above-described chromatographic separation is carried out while maintaining the flow rate of the pump constant, it is necessary to set the discharged head of the pump and the pressure resistance of the packed bed column at or above the maximum pressure reached in the operation. These capacities are made full use of in the step of supplying the feedstock fluid but are excessive in other steps, which incurs fruitless increases in construction cost of the apparatus (plant) and operating cost.
An object of the present invention is to provide an improvement in process control in carrying out a chromatographic process for separating a mixture comprising a plurality of components into fractions enriched in the respective components, the process involving variations of concentration and concentration distribution of components in a packed bed as in JP-A-63-158105.
The concept of the present invention consists in controlling the movement of the feedstock fluid, the desorbent fluid and the fluid in the packed bed by maintaining the discharged pressure of the pump constant unlike the method in which the flow rate of the pump is adjusted to a prescribed value.
The present invention provides a chromatographic process of a type wherein a feedstock fluid containing a plurality of components having different degrees of affinity for an adsorbent is supplied into a chromatographic separation system, in which the downstream end of a bed packed with the adsorbent is connected to its upstream end by a piping (fluid channel) to enable the fluid to circulate through packed bed, and made to flow in the packed bed from the upstream end of the packed bed to its downstream end thereby to form adsorption zones having the concentration distribution of the respective components in the packed bed and then separated into two or more fractions enriched by the respective components, the process comprising the steps of:
(i) supplying the feedstock fluid into the packed bed at its upstream end while withdrawing a fraction enriched in a first component from the downstream end of the bed,
(ii) supplying a desorbent fluid into the packed bed at its upstream end while withdrawing a fraction enriched in a second component from the downstream end of the bed, and
(iii) circulating the fluid in the bed from the downstream end to the upstream end through the piping without supplying or withdrawing any fluid to or from the packed bed, thereby making a mixed zone comprising the first component withdrawn in step (i) and the second component move to the upstream end of the packed bed,
a cycle including steps (i), (ii) and (iii) in any order being repeated while controlling the discharged pressure of a pump delivering the feedstock fluid, the desorbent fluid, and the circulating fluid to be constant.